In magnetic resonance imaging, an object to be imaged such as, for example, a body of a human subject, is exposed to a strong, substantially constant static magnetic field. Radio frequency excitation energy is applied to the body, and this energy causes the spin vectors of certain atomic nuclei within the body to rotate or “precess” around axes parallel to the direction of the static magnetic field. The precessing atomic nuclei emit weak radio frequency signals during the relaxation process, referred to herein as magnetic resonance signals. Different tissues produce different signal characteristics. Furthermore, relaxation times are a major factor in determining signal strength. In addition, tissues having a high density of certain nuclei will produce stronger signals than tissues with a low density of such nuclei. Relatively small gradients in the magnetic field are superimposed on the static magnetic field at various times during the process so that magnetic resonance signals from different portions of the patient's body differ in phase and/or frequency. If the process is repeated numerous times using different combinations of gradients, the signals from the various repetitions together provide enough information to form a map of signal characteristics versus location within the body. Such a map can be reconstructed by conventional techniques well known in the magnetic resonance imaging art, and can be displayed as a pictorial image of the tissues as known in the art.
The magnetic resonance imaging technique offers numerous advantages over other imaging techniques. MRI does not expose either the patient or medical personnel to X-rays and offers important safety advantages. In addition, magnetic resonance imaging can obtain images of soft tissues and other features within the body which are not readily visualized using other imaging techniques. Accordingly, magnetic resonance imaging has been widely adopted in the medical and allied arts.
Several factors impose significant physical constraints in the positioning of patients and ancillary equipment in MRI imaging. Many MRI magnets use one or more solenoidal superconducting coils to provide the static magnetic field arranged so that the patient is disposed within a small tube running through the center of the magnet. The magnet and tube typically extend along a horizontal axis, so that the long axis or head-to-toe axis of the patient's body must be in a horizontal position during the procedure. Moreover, equipment of this type provides a claustrophobic environment for the patient. Iron core magnets have been built to provide a more open environment for the patient. These magnets typically have a ferromagnetic frame with a pair of ferromagnetic poles disposed one over the other along a vertical pole axis with a gap between them for receiving the patient. The frame includes ferromagnetic flux return members such as plates or columns extending vertically outside of the patient-receiving gap. A magnetic field is provided by permanent magnets or electromagnetic coils associated with the frame. A magnet of this type can be designed to provide a more open environment for the patient. However, it is still generally required for the patient to lie with his or her long axis horizontal.
Recently, ferromagnetic frame magnets having horizontal pole axes have been developed. As disclosed, for example, in commonly assigned U.S. patent application Ser. No. 08/978,084, filed on Nov. 25, 1997, and U.S. Pat. Nos. 6,414,490 and 6,677,753, the disclosures of which are incorporated by reference herein, a magnet having poles spaced apart from one another along a horizontal axis provides a horizontally oriented magnetic field within a patient-receiving gap between the poles. Such a magnet can be used with a patient-positioning device including elevation and tilt mechanisms to provide extraordinary versatility in patient positioning. For example, where the patient positioning device includes a bed or similar device for supporting the patient in a recumbent position, the bed can be tilted and/or elevated so as to image the patient in essentially any position between a fully standing position and a fully recumbent position, and can be elevated so that essentially any portion of the patient's anatomy is disposed within the gap in an optimum position for imaging. As further disclosed in the previously mentioned applications, the patient positioning device may include additional elements such as a platform projecting from the bed to support the patient when the bed is tilted towards a standing orientation. Still other patient supporting devices can be used in place of a bed in a system of this type. For example, a seat may be used to support a patient in a sitting position. Thus, magnets of this type provide extraordinary versatility in imaging.
Another physical constraint on MRI imaging has been posed by the requirements for RF antennas to transmit the RF excitation energy and to receive the magnetic resonance signals from the patient. The antenna that receives the signals is positioned near that portion of the patient's body that is to be imaged so as to maximize the signal-to-noise ratio and improve reception of the weak magnetic resonance signals. The antenna that applies RF excitation energy can be positioned in a similar location to maximize efficiency of the applied RF energy. In some cases, the same antenna is used to apply RF excitation energy and to receive the magnetic resonance signals at different times during the process. However, it is often desirable to provide two separate antennas for this purpose.
The antennas are typically formed as one or more loops of electrically conductive material. Such a loop antenna must be positioned so that the conductor constituting the loop extends along an imaginary plane or surface having a normal vector transverse to the direction of the static magnetic field. Stated another way, the antenna must be arranged to transmit or receive electromagnetic fields in a direction perpendicular to the direction of the static magnetic field if it is to interact with the precessing atomic nuclei. This requirement has further limited available antenna configurations and techniques. For example, in a vertical-field magnet such as a ferromagnetic frame magnet having a vertical pole axis, it is impossible to use a loop antenna with the loop disposed generally in a horizontal plane below the body of a recumbent patient. Such an antenna has a normal vector which is vertical and hence parallel to the direction of the static magnetic field. A loop antenna which encircles the patient with its normal vector extending horizontally can be employed. Also, planar or saddle-shaped loops extending in generally vertical planes or surfaces, and having normal vectors in the horizontal direction transverse to the long axis of the patient can be positioned on opposite sides of the patient. However, these antenna configurations do not provide optimum signal-to-noise ratios in some procedures as, for example, in imaging the spine, head or pelvic region.
More recently, quadrature coil arrangements having a combination of planar coil antenna have also been developed. As disclosed, for example, in commonly assigned U.S. Pat. No. 7,701,209, issued on Apr. 20, 2010 to Charles A. Green (hereinafter, the '209 Patent), the disclosure of which is incorporated by reference herein in its entirety, two planar coil antenna are combined in the seat and/or back of a chair that defines a support for a patient. In one example, the coil arrangement consists of a substantially flat butterfly receiver coils arranged in a quadrature mode. First and second coils with perpendicular coil vectors lie in a substantially parallel plane that is parallel to surface of the patient support.
In another example, a quadrature planar coil antenna assembly includes a loop coil antenna and a butterfly coil antenna mounted to a support using a plurality of mounting members. The coil vector of the loop coil antenna is perpendicular to the surface of the support, and the coil vector of the butterfly coil is parallel to the surface of the support. The antenna of the second example is vertically adjustable along the support to allow for imaging of the spine, heart, or other areas of the torso, and may also be adjusted to allow a patient to sit on the antenna so that images of the lower abdomen, e.g., pelvic region or prostate, can be obtained.
However, imaging performed using the previously disclosed quadrature coil arrangements is subject to several limitations. For example, due to the substantially planar configuration of each of the coils, it is necessary to place the target imaging area of the patient as close as possible to the coil in order to produce a higher quality magnetic resonance image. Generally, the target imaging area for a patient is separated from the coils by layers of skin, tissue, and fat. The amount of the separation may vary depending on the physique of a given patient. As such, the quality of images taken using the above described coils have a high variance of quality and, in general, are of low quality.
For further example, imaging of a patient's prostate is still generally performed using an RF coil that inserted into the patient. Such techniques are generally uncomfortable and unpleasant. As such, there is a great demand for an apparatus and method for imaging a patient that is less intrusive while yielding high quality (e.g., high-resolution) results.